Design of Reinforced Concrete Balcony with Dwarf Wall
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| Plan of Balcony Slab |
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| Section through Balcony Slab |
Moment and Shear
Slab Geometry
Span of slab = 1200 + (225/2) = 1312.5 mm = 1.3125 m
Design width = 1000 mm = 1 m
Slab Details
Thickness of slab = 150 mm
Characteristic strength of concrete; fcu = 20 N/mm2
Characteristic strength of reinforcement; fy = 460 N/mm2
Material safety factor; γm = 1.05
Cover to bottom reinforcement; c = 20 mm
Cover to top reinforcement; c’ = 20 mm
Loading details
Slab loading
Dead load
Self weight of slab = 0.15 × 24 = 3.6 kN/m2
Finishes @ 0.6 kN/m2 = = 0.6 kN/m2
Characteristic dead load; gk = 4.2 kN/m2
Dwarf wall loading
Wall is 150 mm hollow block wall (BS 648:1964, Schedule of weights of building materials)
Load per m run (Point load) = 1.52 × 1 = 1.52 kN/m
Characteristic dead load; gk = 1.52 kN/m
Imposed load (Office general use = 2.5 kN/m2 ,BS 6399-1:1996, Table 1)
Characteristic imposed load; qk = 2.5 kN/m2
Design loading factors
Dead load factor; γG = 1.4
Imposed load factor; γQ = 1.6
Moment redistribution ratio; βb = 1.0
Design loads
Dead loads
Slab
Slab load = 1.4 × 4.2× 1 = 5.88 kN/m
Dwarf wall load = 1.4 × 1.52 = 2.13 kN
Imposed loads
Slab
Slab load = 1.6 × 2.5× 1 = 4.00 kN/m
Moment and Shear
Service Moment
Mudl = (0.5 × 4.2 × 1.31252) + (0.5 × 2.5 × 1.31252) = 5.77kNm
Mpoint = 1.52 × 1.3125 = 2.00 kNm
Design Moment
Mudl = (0.5 × 5.88 × 1.31252) + (0.5 × 4.00 × 1.31252) = 8.51kNm
Mpoint = 2.13 × 1.3125 = 2.80 kNm
Design Shear force
V = (5.88 × 1.3125) + (4.00 × 1.3125) + 2.13 = 15.10 kN
Slab Design (Per metre run of balcony)
Using 12 mm main bars and 10 mm distribution bars
Effective depth of reinforcement; d = 150 – 20 + (12/2) = 124 mm
Support moment; m’ = 8.51 + 2.80 = 11.31 kNm/m
Design reinforcement
(3.4.4.4)
Lever arm; K’ = 0.402 × (βb – 0.4) – 0.18 × (βb – 0.4)2 = 0.176
K = m / (d2 × fcu) = 0.036778 < 0.176
Compression reinforcement is not required
(3.4.4.4)
z = min((0.5 + √(0.25 – (K / 0.9))), 0.95) × d = 117.8 mm
Area of reinforcement designed; Asreqd = m / (z × fy / γm) = 219.70 mm2/m
Minimum area of reinforcement required; Asmin = 0.0013 × h = 195 mm2/m
Area of reinforcement required; Asreq = max(Asreqd, Asmin) = 219.70 mm2/m
Provide 12 mm dia bars @ 200 mm centres
Area of main reinforcement provided; Asprov = 566 mm2/m
Provide 10 mm dia bars @ 200 mm centres
Area of distribution reinforcement provided; Asprov = 393 mm2/m
Shear Check
Maximum allowable shear stress; vmax = min(0.8 × √(fcu), 5) = 3.58 N/mm2
shear stress; v = V / (b × d) = 15.10 ×103 / (1000 × 124) = 0.122 N/mm2
Shear capacity of Slab; vc = (min(fcu,40)/25)1/3×0.79×min(100×Asprov/(b×d),3)1/3×max(400/d,1)1/4/1.25
Shear capacity of Slab; vc = 0.756 N/mm2 > 0.122 N/mm2
Shear Capacity Okay
Check deflection
Basic span/d ratio = 7
Kudl = 0.25
Kpoint = 0.33
Adjusted basic ratio = Basic ratio× (Mudl + Mpoint × Kudl/ Kpoint)/( Mudl + Mpoint )
Adjusted basic ratio = 7× (5.77 + 2.00 × 0.25/ 0.33)/( 5.77 + 2.00 ) = 6.56
Design service stress; fs = 2 × fy × Asreq / (3 × Asprov × βb) = 119.04 N/mm2
Modification factor; k1 = min(0.55+(477N/mm2-fs)/(120×(0.9N/mm2+(m/d2))),2) = min (2.38, 2.00)
Modification factor; k1 = 2.00
Allowable span to depth ratio; = 6.56 × k1 = 13.12
Actual span to depth ratio; L / d = 1312.5/124 = 10.58
L/d ratio Okay
Check Cracking
Clear spacing of bars = 200 – 12 = 188 mm
3d = 3 × 124 = 372 mm
47000/fs = 47000/119.04 = 394 mm
Bar spacing Okay
Anchorage length
Ultimate anchorage bond stress, fbu = β√fcu = 0.5√20
fbu = β√fcu = 2.24 N/mm2
L = (0.95fy) × f/ (4 fbu)f = (0.95×460×12)/ (4× 2.24) = 585.26 mm
L = 0.58526 m
However, ISTRUCTE detailers’ manual recommends
L = (1.5 × span) + 0.1125 = (1.5 × 1.3125) + 0.1125
L = 2.08125 m
Also L = 0.3 × span preceding cantilever span
In this case we take 4.5 m
L = 0.3 × 4.5 = 1.35 m
Therefore we adopt L = 2.08125m
i.e. 2.1 m from the centerline of the support
Good post.
ReplyDeletehttp://structuraldesignbs.blogspot.com/
Thank you very much, Bandula Prasad
DeleteHi, I really liked your indepth analysis of reinforcement related to structural stability wrt cantilever beam and slab
Deletefor balcony. Could you please also assess the following case I posted on Quora.
How do I extend a balcony of 1.2x2.5m of a preconstructed apartment using cantilever beams?
It is a third floor. Column size is 9x12 inches. I am going to use 6 top bars of 16 mm and 3 bottom bars of 16mm, curtailing 2 top bars at 0.6m with the main slab bars of 12mm and distribution bars of 10mm. Stirrups bar of 8mm with spacing 4 inches center to center. cover block of 30mm with hook angle 135 degrees and angling downwards. slab thickness at support 6 inches
As it is preconstructed apartment, I could not extend the bar back, however I have reclaimed around 0.6m beam and slab bars after demolishing beam and slab. Will this be enough for lapping length of slab and development length of cantilever beam. I plan to create a brick wall (4 inches, 9 feet ceiling height) to include this space inside to be used as a kitchen counter of 2 feet and overhead storage. What modifications I can perform to ensure safety.
Total reinforcement area is now 3feet 6 inches cantilever, column(9″x12″), 2feet overlapping length.
I really liked your analysis. Could you please also assess the following case for a precasted slab where only cantilever balcony has to be made. I posted this on quora.
DeleteHow do I extend a balcony of 1.2x2.5m of a preconstructed apartment using cantilever beams?
It is a third floor. Column size is 9x12 inches. I am going to use 6 top bars of 16 mm and 3 bottom bars of 16mm, curtailing 2 top bars at 0.6m with the main slab bars of 12mm and distribution bars of 10mm. Stirrups bar of 8mm with spacing 4 inches center to center. cover block of 30mm with hook angle 135 degrees and angling downwards. slab thickness at support 6 inches
As it is preconstructed apartment, I could not extend the bar back, however I have reclaimed around 0.6m beam and slab bars after demolishing beam and slab. Will this be enough for lapping length of slab and development length of cantilever beam. I plan to create a brick wall (4 inches, 9 feet ceiling height) to include this space inside to be used as a kitchen counter of 2 feet and overhead storage. What modifications I can perform to ensure safety.
Total reinforcement area is now 3feet 6 inches cantilever, column(9″x12″), 2feet overlapping length.
Good Post
ReplyDeletewww.structuraldesignbs.blogspot.com
Hi. What happened if the balcony need to drop let say 100 mm to prevent rain water from getting into the main building. When should a cantilever slab be supported by a cantilever beam? Do you have any materials on pitch/slope roof slab design and detailing? Thanks.
ReplyDeleteHi Nicholas Marie, if the balcony need to drop let say 100 mm, that part is treated as a line load or more correctly as a point load but expressed as KN/m run of balcony. There are no hard and fast rules when a cantilever slab be supported by a cantilever beam. It all depends on the desire of the architect. As for materials on pitch/slope roof slab I will communicate with you later. Best wishes.
ReplyDeleteCan you elaborate more on this please? How is the top bar (T1) detailed due to the drop in level? Thanks.
ReplyDeleteIf there is a drop in the level of the slab, then the conditions have changed. The intervening beam(i.e. the beam carrying balcony will be under torsion. The connections, the beam and the slab should be designed for torsion.
DeleteAdequate anchorage should be provided for the bent bars.
What is 3.4.4.4?
ReplyDeleteSorry for the mix-up. It should have been stated in full that is Clause 3.4.4.4 of BS 8110, Part 1. That is the reference clause.
DeleteWhat is 3.4.4.4?
ReplyDelete2.1 mtr width and length 7.0 mtr sit-out slab,with 2 side wall support. one column provided for supporting slab that is ( from wall to center of column distance 1.25 mtr and other side from wall to center of column 3.35 mtr so tell me how to construct this slab
ReplyDeleteI,ve been trying to understand the layout of the sit-out slab. Questions are:
ReplyDelete1. Sit-out should mean balcony?
2. The thickness of the slab is not given.
3. You might need to provide the sketch of the slab, showing the walls and columns positions.
You can send mail to monomaklass@gmail.com
1) This area used as a sit-out above this no room, no balcony( empty area)
ReplyDelete2) 2.5 or 3 inch sir u can propose this thickness
3) i can send to your mail
thanks sir
Can get bar bending schedule?
ReplyDeleteGreat information, Thank you for sharing and keep posting Balcony Safety Nets In Hyderabad, i found Ram Tej Safety Nets are providing free installation, Best Quality & Strong Nets, 24/7 customer support and annual maintenance with ease.
ReplyDeleteThank you for sharing your thoughts and knowledge on this topic. This is really helpful and informative!
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Thank you very much.
DeleteThanks for the valuable info. But I wanna ask u what is 1.52 in dwarf wall? Thanks again
ReplyDeleteGood post. One small question, if the anchorage length is less that that suggested by IStructe, say 1xcantilever span, will it effect the overall stability of the cantilever? (Assuming backspan main steel is orthogonal to the main steel of balcony.)
ReplyDeleteIt will not affect the stability overall stability, but your structure might be exceeding the factor of safety. Orthogonal reinforcement is independent of the other direction.
DeleteGood post. What can be the consequences if the anchorage length is say 1xcantilever span? (Assuming main steel is orthagonal in back span to main steel of balcony)
ReplyDeleteReply is same as above.
DeleteGood information. How do I design a two span sloping beams with an overhung of 1.2m. the beams placed 6m centers both ways, the slab 175mm thick... Carries curved steps 300mm high and 1000mm wide all through
ReplyDeleteFor gentle slopes, the beams are designed as horizontal beams. the same way you design staircases. By curved do you mean helical beams.
DeleteThank you, this will be useful in designing a portion of a 2- meter cantilever bedroom of a condominium villa.
ReplyDeleteThank you, sir. This can be a guide in the design of a 2- meter cantilever bedroom of a condominium villa.
ReplyDeleteNice post. Thanks for sharing the valuable information. it’s really helpful. Who want to learn this blog most helpful. Keep sharing on updated posts.
ReplyDeleteOnline 3d rendering services
How to determine cantilever slab load distribution to beam???
ReplyDeleteFor a one cantilever like the one I have done, it is one way. Same for cantilver bounded by two or three beams.
DeleteWhat will be cantilever slab load distribution to beam??
ReplyDeleteSame as above.
DeleteGood post
ReplyDeletePlease why is lever arm limited to 0.95d
ReplyDelete